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Analysis of a Mars-Stationary Orbiting Microwave Power Transmission System

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Analysis of a Mars-Stationary Orbiting Microwave Power Transmission System NASA Technical Memorandum 101344 Analysis of a Mars-Stationary Orbiting Microwave Power Transmission System Kenwyn J. Long Lewis Research Center Cleveland, Ohio July 1990 ANALYSIS OF A MARS-STATIONARY ORBITING MICROWAVE POWER TRANSMISSION SYSTEM Kenwyn O. Long National Aeronautics and Space Admlnistration Lewis Research Center Cleveland, Ohlo 44135 SUMMARY An efficient Mars-stationary orbiting microwave power transmission system can fulfill planetary exploration power requlrements. Both nuclear power gen- eratlon and photovoltaic energy conversion have been proposed for the orbiting power source. Thls power is to be converted to RF energy and transmitted to the surface of the planet, where it is then to be converted to dc power. An analysis was performed for example systems at 2.45 GHz, where rectenna technology Is currently well developed, and at several higher frequencies in the 2.45- to 300-GHz range, where small antenna requirements make the system more viable. cO Thls analysis demonstrates that while component efficiencies are high at ! I,l 2.45 GHz, antenna dlmensions required to deliver a desired power level to the planetary surface are unachievably large at that frequency. Conversely, as the operating frequency is raised, component efficiencles fall, requirlng an increase in source power to achieve the desired rectified power level at the surface. Efficiencies of free electron lasers operating in the 30- to 200-GHz range are currently low enough to offset any advantage derived from the highly directional laser output beam. State-of-the-art power transmitters at 20 to 30 GHz have moderate compo- nent efflciencles and provlde fairly high output power levels. In this fre- quency range, the antenna dimension requirement is approximately one-tenth that of a 2.45-GHz system. These factors, along with current rectenna development efforts at 20/30 GHz, make the 20- to 30-GHz range most desirable for Inltial microwave power transmlssion system development. Both parabolic and phased-array transmitting antennas were investigated. The number of phased-array elements required to prevent grating lobe interfer- ence at these high frequencies may present significant phase control problems unless an appropriately dimensioned phased-array-fed reflect:or system can be designed. Development of rectenna technology at 20 GHz and above is of prime impor- tance in achieving realistic transmitting antenna dimensions. Large, high- gain transmitting antennas are necessary to p_'ovide a power flux density high enough to meet the threshold power density criteria of the _ectenna dipole elements. INTRODUCTION To determine the feaslb111ty of provtdlng efficient RF power transmlsslon from a Mars-statlonary (areosynchronous) orbit to the surface of the planet, an assessmentwas made focusing on RF propagatlon In the 2.45- to 300-GHz range. In order to provlde a rectlfled load power of I0 to 100 kW at the surface, gen- erated power of approximately lO MW to 1.5 GW was considered. The proposed orblting system conflguration, described In the first sec- tlon, provides for power generat|on by either photovo]talc array or nuclear reactor, the conversion of the dc output to RF, and the subsequent propagation of RF energy from the orbltlng array to the Martian surface. On the planet, a rectenna array wl]l convert RF to dc power to be distributed for planetary power needs. The state-of-the-art of component technologies applicable to the orbiting microwave power transmission system Is presented in the second section. Contlnuous-wave power transmitters within the desired frequency range were assessed with regard to operating efficiency, peak power output, and magnetic field requirements. The thlrd section derives the total efficiency of the energy conversion chain from dc to RF In orbit through RF to dc on the planetary surface for a 2.45-GHz system, which Is the sole frequency for which dlpole-element rectenna technology has been developed to date. Because it would not be possible to accurately project rectenna efficiencies for higher frequencies, the efficiency of the energy conversion chain is calculated only up through the Incident RF power at the surface of the rectenna. Tradeoffs between component efficiency and transmitting antenna requirements were considered for each of several representative frequencies within the 2.45- to 300-GHz frequency range. Receiving antenna criteria were determined from desired received power levels and rectenna element power density threshold at 2.45 GHz. Recommendations are presented for research into developlng technologies which may afford enhanced vlabIllty of the proposed microwave power transmls- slon system. MARS-STATIONARY ORBITING MICROWAVE POWERTRANSMISSION SYSTEM System Design Figure 1 Illustrates the proposed design of the Mars-stationary orbltlng microwave power transmission system. A nuclear reactor or photovoltaic array wlI] be placed in orbit about 17 000 km above the surface of the planet. A power distrlbution array will then transfer the dc power from the source to a power transmitting device, or to an array of devices, for conversion from dc to RF energy. A transmitting antenna array will then propagate the RF energy through the Martian atmosphere down to an array of rectennas on the surface of the planet. Thls rectenna array will convert the received energy from RF to de, where it will be distributed as dc power for planetary use. For simplification, in this analysls the receiving antenna array Is taken to be centered at the subsatelllte point (i.e., at a point on the Martian equa- tor). Transmission to surface points other than this would Increase the trans- mission path length and atmospheric attenuatlon. Radlo Frequency Energy Attenuation In Martian Atmosphere The physical and chemlcal properties of the Martian atmosphere were asses- sed to determine extinctlon effects on RF transmissions from Mars-statlonary orbit down to the surface of the planet. These properties include atmospheric composition, absorption characteristics, and scattering effects due to wind- borne dust particles. The composition of the atmosphere is approximately 95 percent carbon dioxide (C02), 2.7 percent nitrogen (N), 1.6 percent argon (Ar), and trace gases of oxygen (02), carbon monoxide (CO), neon (Ne), krypton (Kr), xenon (Xe), and ozone (03). Relatlve concentrations of these molecules are [COl:[CO 2] = 10-3 [02]:[C02] = 1.3xlO -3 [03]:[C02] : 2xi0-6 Atmospheric pressure on Mars ranges from 5 to 8 mbar as compared with the l.Ol3 bar Earth value. Water vapor appears to be a highly variable component of the Martian atmosphere. The north polar cap is believed to be water ice, which partlally sublimates in the summer to release water vapor into the atmosphere. Although water vapor has been detected in the Martian atmosphere, the concentrations of other species of hydrogen and oxygen (H, H2, OH, and H202) are apparently below the detection threshold of the Vlklng lander instruments. Since the concentra- tion of water vapor varies widely both seasonally and geograph|cally, it is not possible at thls time to assess the degree of attenuation due to that molecule. However, since the overall concentration appears to be quite low, attenuation at the two H20 absorption lines within this band (22 and 180 GHz) should be minimal. Although molecular oxygen, 02 , has absorption lines extending from 53.5 to 65.2 GHz and at I18 GHz, this molecule exists only in trace quantities. Atten- uation at these frequencies should therefore be negligible. The majority of the absorption lines for CO 2, the predominant atmospheric constituent, fall within the infrared region between 20 and 150 THz (15 and 2 _m). This is well above the l- to 300-GHz (3xlO -l to IxlO -3 m) range of investigation. It should be noted, however, that complete absorption data for the Martian atmosphere are not currently available because the Viking lander data have not been completely analyzed to date. Estimates of Martian windborne dust particles range in diameter from 0.I to 30 pm (ref. I). Scattering of transmissions in the I- to 300-GHz range is not expected to be of concern because the wavelengths correspondlng to these frequencies (3xi0 -I to IxlO -3 m) are much larger than the dust partlcle diameters. However, heavy accumulatlon of dust on surface-based rectenna ele- ments Is expected to cause scattering of Incldent RF radlatlon. Orbiting Power Source To meet the power requirements of Mars planetary exploration by the pro- posed microwave power transmission system, it has been estimated that the orbiting power source must be able to gener3te between 1MW and 1GW of dc power. Nuclear generators and photovo]talc arrays have been proposed as power sources. Nhile each has its respective merits and disadvantages, it is not within the scope of this inltlal study to assess these power sources for use In the power transmlsslon system. TECHNOLOGY APPLICABLE TO ORBITING MICROWAVE POWER TRANSMISSION SYSTEM Direct Current to Radio Freauency Conversion and Transmission For efficient transmission to the surface of Mars, the dc power generated at the orbiting source must be converted to RF. This dc to RF conversion and transmisslon may be achieved by any one of several devices which, according to the associated phase velocity, are categorized as slow-wave or fast-wave devices. Selection of the appropriate power transmitter is a function of oper- ating frequency, gain requirements, noise limitations, and efficiency. Slow-wave devices I (phase velocity < c). - Slow-wave devices include mag- netrons, amplitrons, klystrons, and travellng-wave tubes. Magnetrons can provide up to I kN of peak power at 2.45 GHz with an effi- clency of 0.7 percent. They are relat_ely low in weight, but have a low degree of frequency stability. Although amplitrons offer high efficiency (~80 percent) and need no active cooling, they have a low-gain, low-power output with a high degree of noise. These devlces are constrained to frequencies less than 10 GHz.
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